It was a classic stakeout: for some time government agents had the Bar Venezia in Stigliano, a small town in Italy's deep south, under surveillance. This February, as Salvatore, oblivious of the trap about to be sprung, came out into the street the team moved in with cool efficiency. Seconds later, without a struggle or a shot fired, Salvatore was taken into custody.

The crime: dealing a 100-lire bag of popcorn without a scontrino (cash register receipt). The penalty: a 300,000-lire (about $240) fine for the bar owner who had sold the popcorn, and one of 33,000 lirefor Salvatore - who had to be bailed out by his father, seeing that he is only 7 years old.

According to Italian law, not only must a shopkeeper issue a receipt but those buying goods must be able to produce it on demand.This draconian system was first introduced in 1983 by the then-Minister of Finance Bruno Visentini as a desperate measure to persuade Italy's 4 million or so shopkeepers - some of them regularly declaring incomes lower than their lowest-paid workers - to pay their share of taxes. The law sparked off a national shopkeepers' strike and a political crisis that nearly brought down the government.

The body entrusted with enforcing the law is the Guardia di Finanza, or Fiscal Police, a military-style force in distinctive gray uniforms with yellow trim.The Guardia is responsible for customs, excise and coast guard duties (it has special alpine and nautical training schools) and enforcing internal tax laws. Some tax-dodgers and smugglers, notably the Mafia, are well-equipped and extremely nasty, and the corps has an impressive array of hardware, including patrol boats armed with cannons, helicopters, planes and heavy machine-guns.

The Guardia are nothing if not zealous, but in the Salvatore case, even Minister of Finance Rino Formica admitted that they had gone too far, describing it as "an isolated incident," and maintaining that the service "was certainly not targeting children."

Hardly were the minister's words out of his mouth when the Guardia was in action again, this time nabbing a 14-year-old high school student near Rome who left 200 lireon the counter of a stationer's for two sheets of foolscap without waiting for a receipt because he was late for a French test.

The fiscal crime busters' true moment of glory came at the end of last month in Rovigo, a town southwest of Venice, when 2-year-old Enrico and his mother were stopped outside a bar, brazenly receiptless with a 100-lirechocolate bar. Bracing itself against the inevitable storm of public protest and press reaction, the Guardia issued an official justification, containing an almost audible sigh, saying that the resulting "fine was directed at the mother and obviously not the child. One can only hope that if, in the future, a dog-food seller fails to issue a receipt that it will not be claimed that the dog was booked."

As from the end of March, in addition to shops, restaurants, bars and hotels, many other traders are now obliged to give receipts: these include barbers; car, equipment and videocassette rental firms; and hirers-out of deck chairs, beach umbrellas and pedalos. My barbers seem to be taking this on the chin, and have dug out their old receipt books (which they had to use for a short time some years ago, before barbers were, for some unknown reason, again exempted). One of their colleagues in Venice has, however, shut up shop and retired in disgust.

Certain categories remain exempt, such as newspaper vendors and booksellers, tobacconists and gas stations. But it is the government's intention to widen the net to cover taxis, street traders, farmers and even gondoliers.

So far receiptless tourists and foreigners seem to have been spared the attentions of the Guardia, an impression supported by the British consulate in Venice and the U. S. Embassy in Rome, both of which say they have yet to deal with such a case. However, it is wise, especially if you buy something of any value, to make sure you have a proper written receipt (ricevuta fiscale) to show the Guardia, on request, at the border or airport.

NEEDLESS to say, the Guardia are not the most popular of Italy's public servants. But without them the country's budget deficit, already predicted to reach nearly $130 billion this year, would undoubtedly be even higher.

Nor can it be said that they lack a sense of humor. A couple of years ago Mabor, their redoubtable Alsatian sniffer dog at Venice airport, cornered a traveler on an internal flight from Rome. Declining to prosecute the young man for possession of drugs (he was carrying less than a gram of cannabis), but finding that he could not produce a receipt to prove he had bought it in Italy, the Guardia assessed the value of the product and charged him 1,800 lirefor unpaidvalue added tax.